Method of detecting frequency modulated waves



arch 12, 1946. ZEGLER 2,395,369

METHOD OF DETECTING FREQUENCY MODULATED WAVES Filed July 16, 1942 2Sheets-Sheet 1 ATTORNEY.

M. ZlEGLER ala'ch 12, 1946.

METHOD OF DETECTING FREQUENCY MODULATED WAVES 2 Sheets-Sheet 2 FiledJuly 16, 1942 v n BN Patented Mar. 12, i946 UNHTED STATES PATENT @EFECE"METHQD F DETECTING FREQUENCY MODULA'EED WAVES Application July 16, 1942,Serial No. 451,186

7 Claims.

The present invention relates to the detection of frequency modulatedwaves. Neglecting certain possible constant phase displacements, afrequency modulated wave may be considered as a sinusoidal function ofthe sum of two angles, one of which is the central angle equal to theproduct of the constant central frequency by the time, and the other ofwhich is the variable angle, hereinafter called modulation angle, whichrepresents the impressed phase variation correspond ing to theintelligence. For the normal customary case, this variable phase isproportional to the integral of the intelligence. The method of thepresent invention, however, makes its equally easy to receive moreelaborately modulated waves,

the variable phase of which can have, with respect to the intelligence,a wide variety of relationships which can be comprised under the generalterm of modulation characteristic. Thu-s, such a Wave may be written Esin (wt+ i), with 411:? (s) where \l represents the varia le ormodulation angle, 8 is the intelligence, 5 the modulationcharacteristic, to is the central frequency and of the central angle.

As is well known, classical frequency modulation receivers are based onthe selective properties of L. C. circuits which make it possible tochange frequency modulation into amplitude modulation. From theamplitude modulated signal thus obtained, the intelligence is thendetected according to the classical amplitude modulation detectionmethod. Known frequency modulation receivers normally consist of a radiofrequency stage, one or two frequency converter stages, two 'wide bandamplifier stages, two limiter stages, a discriminator and detector,followed by the ordinary amplifier and output stages.

Now it can be shown that the highest value of 1 01 that will be found inthe practice of high fidelity frequency modulation, is of the order of2,000 radians, which corresponds, for a modulating frequency of 30c./s., to a frequency excursion of '75 kc. It will readily beappreciated therefore. that by reason of this large frequency excursion,the classical receiver, in order to pass the whole spectrum of thefrequency modulated signal, must be equipped with intermediate frequencyampli-- fiers of large bandwidth, of the order of 200 kc. Consequently,in the classical receivers, it is nec-- essary to use special wide bandamplifiers working on relatively high frequency (5 mo.) and with arelatively low amplification per stage (an amplification of the order of20).

It is the principal object of the present invention to provide a new andnovel method of de tecting a frequency modulated wave.

Another object of the invention is to provide a method of demodulating awave varying as a inusoidal function of a central angl and a modulationangle.

A further object of the invention is to provide a device for detecting amodulated wave varying as a sinusoidal function of the sum of a centralangle and a modulation angle in which device the need for special wideband amplifiers is obviated.

These and further objects of the invention will appear as thespecification progresses.

In accordance with the invention I obtain accurate detection of afrequency modulated wave with a considerably simplified apparatus, bysubjecting the modulating component of the incoming wave to an operationwhich I have for convenience designated shrinkage, and which is effectedby impressing on the wave a certain phase shift angle 11 1' whichcombines with the modulation angle n to give a difference angle we,which, as hereinafter will be made clear, can be made very small andproportional to the modulation angle, so that said difference angle willfinally reproduce the same modulation characteristic with respect to theintelligence as the modulation angle does. The result of producing thesmall difference angle is to compress or shrink the frequency excursionso that the spectrum does not extend further from the central frequencythan a value equal to the highest modulating frequency. This means thatthe bandwidth required is exactly the same as in ordinary amplitudemodulation receivers, and further, that the intermediate frequencyitself may be the same as that commonly used in such receivers, forexample 450 kc.

The wave treated as bove described by passing it through a phaseshifting device and comprising the difference angle in lieu of themodulation angle, is subjected to on or more steps of heterodynefrequency changing to produce an intermediate frequency wave varying asa sinusoidal function of the sum of the intermediate frequency angle andthe said difierence angle. The intermediate frequency wave so producedis then impressed on a phase comparison device, where it is compared ormixed with a locally generated oscillation Varying as a sinusoidalfunction of the intermediate frequency angle. The output of the phasecomparison device comprises an electrical quantity which is proportionalto the difference angle. The said quantity, after pre-amplification, isimpressed on a restoring device, Where the inverse operation of themodulation characteristic is performed, as for example, adifferentiation by means of a simple inductance when .a classicalfrequency modulated wave is being treated, in which the modulationcharacteristic is an'integration. This restoring device thus convertsthe impressed quantity into a second quantity which is directlyproportional to the intelligence itself. Said second quantity may, forexample, be impressed on a loud speaker or the like, whereby theintelligence will be audibly reproduced.

Thus it will be clear that the novel method of detecting frequencymodulated waves, according to the present invention,.is basically quitedifferent from the known methods.

As will be more clearly explained below, these results depend on thepossibility of maintaining the difference angle proportional to themodulation angle and yet smaller than radian, and these conditions canbe satisfied in practice by impressing on the phase shifting device aquantity derived from the phase compar son device and univocallvdependent on the output thereof.

As will hereinbel w be made clear, my novel method permits faithfuldetection of the inte11i-' gence inscribed in the frequency modulatedwave to be achieved independently of the'amn itude of the wave and alsoirrespective of non-linearity of arts of the circ it.

The invention will be described in greater detail with reference to t eappended draw ngs forming part of the specification and in which Fig. 1is a diagram illustrating the demodulating effect of a p ase shiftingdevice on a frequency modul ted wave.

Fig. 2 is a diagram illustrating the manner of securin automaticadiustrnent of the operating con itions of the phase s iftin device sothat the dem dulating action shall be nearly equal to the?1 modulationinscribed on the incoming wave an Fig. 3 is a circuit diagr m of areceiver including a phase shift ng device and desi ned to operate onthe method of detection of the present invention.

Fig. 4 illustrates a phase shift ng device and its arrangement in thereceiver circuit of Fig. 3.

From the fore oing explanations it will be evident that a means forimpressing on the incomin wave a phase shift of the kind desired is anessential and necessary part of a receiver adapted to operate on mynovel method of detection. The specific construction and design of sucha means is no part of the present invention, since apparatus capable ofgiving the desired results is at present available. although it has nothitherto been so employed. However, for the better understanding of thepresent method. it is thought advisable to explain briefly what is heremeant by a phase shifting device.

A phase shifting device, represented diagrammatically by the rectangle Tin Figure 1, comprises input terminals A, output terminals B and controlterminals C, and has an internal arrangement such that if a tensionv1(t) is applied to the terminals A, there will appear at the outputterminals B, tr seconds later, a tension p being a proportionalityfactor and tr being at each instant proportional to an electricalquantity applied to the controls terminals C. Apparatus suitable for useas a phase shifting device in the present invention, has been describedas a phase modulation tube in Ed. Labins U. S. patent application Ser.No. 436,374, filed on March 26, 1942, now U. S. Patent No. 2,372,210,issued March 27, 1945, and the use thereof in a circuit arrangement inaccordance with the invention is illustrated in Fig. 4, later to be morefullyreferred to.

Now, if as a measure of the time tr, the phase of an oscillatory wavehaving an angular velocity 0 be taken, the efiect of passage through thephase shifting device will be evidenced as a phase shift 11/: (equal toQtr) and if a current 1' be applied to' the control terminals C, it canbe shown that the practical formula for such a phase shiftins device isWhere )4 is the phase shifting constant of the device in radians permilliampere.

As is apparent from the above cited prior application of Ed. Labin, sucha device has heretofore been used for the modulation of the frequency ofa wave. The present invention provides a method in which such devicewill be used to perform the inverse action.

Referring again to Figure 1, it is supposed that there is applied to thephase shifting device a Wave E1 sin wt+1('t) Where. 01(1.) represents avariation of phase with time. ample, be of the form gl/io sin Qt.

The control or phase shifting current i applied to the terminals C, ismade equal to E2 sin (wt-i-yW-ybr) =E2 sin (wH-ybi-dgbi) E2 sin (wt-Hts)If a=1, the wave has no more phase variations;

if a is slightly less than 1 it has a phase variation which is smallcompared with the original varia-.

tion. That is to say, a shrinkage of the phase difference has beenproduced to the value emu-am This means that a shrinkage of theexcursion has also been achieved, which reduces the spectrum of the Waveto a relatively narrow frequency band.

The next step will now be so to arrange matters that, in the event that1/4 varies in an unknown 'manner, 51 shall be automatically maintainedat a value nearly equal to it, (11 1), so that as a consequence, thedemodulating current 1', equal to rrsL' x x will be proportional to themodulation angle l -z, in order that said current will carry orrepresent the intelligence, while the shrunk angle its will berelatively small.

It will now be shown how this result can be achieved.

As shown in Fig. 2 the output oscillation E2 sin (wt-H1111) of the phaseshifting device T is mixed with a local signal Er. cos wt in a phasecomparisondevice M, The mixing of the oscillations in the rind) may, forexdevice M produces in Well-known manner a difference frequency currentproportional to The other frequencies produced by the mixing can readilybe filtered out as they are of high frequency, and will not beconsidered in the present description. By analogy with the operation ofinverse feedback in an amplifier on the input of which a large signal isapplied which is reduced automatically to a small value by subtrac tionof a tension derived from the output, those skilled in the art willappreciate that by thus applying, as in the figure, to the phaseshifting device, the output of the phase comparison de vice pro ortionaltoasinusoidal function of the difference angle is, the desired phaseshifting current i proportional to the modulation angle canautomatically be obtained. Evidently the output current derived from thephase comparison device and applied to the phase shifting device must beunivocally correlated with ts, so that this difference angle must notreach values exceeding This condition can readily be satisfied by sochoosing the constants of the arrangement that ts is sumciently smallfor the sine of the difference angle to be approximately equal to theangle itself.

From the feedback theory it can be deduced that, in the arrangementdescribed, we have 31/1 z ybi (1) and 11d tl/i/A (2) when A is made verymuch greater than 1 and do is less than /4 radian, A being the number ofradians phase shift corresponding to /ezl radian.

Now A can be written equal to xii where i1 is approximately thevariation corresponding to an increase he of one radian in the platecurrent of the mixer tube constituting a phase comparison device M. Itis readily possible to make x of the order of hundreds of radians permilliampere and ii of the order of milliamperes, so that a large valueof A can easily be obtained in practice and Formulae 1 and-2 applied.That is to say, that the difference angle 111d can be made proportionalto the modulation angle 1h. Hence the output of the phase comparisondevice will also contain a term proportional to the intelligence.

If, byway of example, it be supposed that A=8,000 and that 1/1 has avariation of from 0 to 2,000 radians, the variation of he will be only2,000/8,00O= radian, so that variations of \//i of the order supposedand even more, can be tolerated. This is an essential property of thenovel method. Not only does it allow these phase fluctuations of forexample 2,000 radians to be considered admissible, but it enables alarge frequency excursion to be reduced to a very low value, which makessuperfluous the use of the special wide band amplifiers necessary inclassical receivers.

The arrangement shown in Figure 2 has no selective properties. Suchproperties can be added by including a selective I. F. amplifier which,owing to the shrinkage of the excursion need be of only narrowbandwidth.

It is further desirable to limit the phase shift ing current i so thatthe range of shrinkage does not extend further than the maximum phaseexcursion of the incoming modulated wave. In practice this limitation isobtainable by using the natural properties of an amplifying tube.

In Figure 3, I have shown a connection diagram for a receiver capable ofoperating according to the method of the present invention. In thisfigure the various elements have been defined, for clarity, byrectangles of broken lines. The circuit is shown as adapted for thereception of a frequency modulated signal E1 sin (wt-Hm) which isimpressed on the antenna AE and is transmitted by way of a selectivecircuit K to the input A of the phase shifting device T describedhereinafter. Phase shifting device T also acts as an M ordinaryamplifier tube and develops at the output terminal B a nearlydemodulated'F; M. wave E2 sin (wi-I-gbd) which is passed through thefrequency changer FC to convert the central frequency to 450 kc./sec.The signal then passes through a common selective intermediate frequencyamplifier IFA of a bandwidth determined by the fidelity desired. Fromthe amplifier IFA the signal passes to the mixer tube MT operating r asa phase comparison device, and in which the I. signal is mixed with alocal oscillation Er. cos wt of also 450 kc./sec. generated by the sametube MT and the associated circuit indicated at L0. The direct and lowfrequency tension on the plate of the mixer MT are amplified by thedirect voltage amplifier PA, the plate current of which varies thereforeproportionally to il d and is used as the demodulating current i byapplying it to the control terminals C of the phase shifting device T.The said plate current is also applied to a restoring device RZ, in thiscase consisting of a self-inductance L, on which a voltage that is avoltage proportional to the previously integrated intelligence iscreated. This voltage is then transmitted to the grid of a power outputtube PO and thence, if desired, to a loud-speaker LS.

As above noted a suitable phase shifting device T is described in patentapplication Ser. No. 436,- 374. Such a device and its embodiment in acircuit arrangement in accordance with the present invention is shown inFig. i. The device comprises a thermionic tube provided with anindirectly heated cathode to generating an electron beam 4|, 2. controlgrid @2 connected to input terminal A and controlling the intensity ofthe a generated electron beam, and an accelerating plate 43 providedwith a hole 45 through which the electron beam penetrates into a transitzone between plate 43 and a diaphragm t5 the central hole 56 of whichforms the point of origin for the electron beam with respect to a phaseshifting zone included between said diaphragm t5 and a collectorelectrode -ll'. Output terminal B of phase shifter T is connected to thejunction point between collector electrode 3'! and an externalresistance 38. A load impedance 49 tuned to the central frequency of thewave to be received is connected in shunt with resistor 8 said loadimpedance being shown in Fig. 3 within the block FC.

The transit zone included between accelerating plate 33 and diaphragm isdesigned to control the velocity and trajectory of the electron beambefore entering the phase shifting zone and may comprise an electroniclens system or any other electron velocity filter of the known type. In

the embodiment of phase shifter T shown in Fig. 4, electron beam 4|describes a helical path controlled by an electric lens system not shownin the drawings. It should be noted, however, that these velocityfiltering means are only needed for low velocity electrons. a

As can be observed from Fig. 4, the phase shifting zone includes anassembly of two serially connected coils 5G and 5| which creates aretarding or phase shifting field, so that the current applied tocontrol terminal C determines the phase shift or retardation impressedon the electrical magnitude represented by the intensity modulatedelectron beam 4 I.

It has, moreover, been shown that when the factor A is sufficientlylarge, that is to say, when there is sufficient amplification andsufiicientsignal strength, 1, l/ z', and this means that or hence holdseven when the amplifier used is not linear,

so that in such cases as well a faithful reproduction of theintelligence is assured. From the negative feedback theory, it alsofollows that local noises are considerably reduced.

In the foregoing discussion it has been tacitly assumed that it waspossible to produce in the local oscillator LO, a local oscillation, thefrequency of which would always remain in step with that of the centralfrequency of the incoming wave or with the frequency convertedderivative of the central frequency. Such an'assumption is consideredjustifiable, since, in my co-pending U. S. application on Method forsynchronising a plurality of oscillations, Ser. No. 450,584, filed July11, 1942, I have disclosed one Way in which the said result may beensured. Moreover, the method of detection of the present invention isindependent in itself of the particular means or process used forsecuring the synchronism of the local and central frequencies. Similarremarks apply if the incoming wave is of the more general typehereinabove referred to.

Although I have described my novel method of detection with particularreference to certain specific examples and certain theoreticalexplanations I do not inmnd to be limited thereto, and it is to beunderstood that I may introduce various modifications in the methodwithout thereby departing from the spirit and scope of the presentinvention as defined in the accompanying claims.

I claim:

1. A method of detecting a frequency modu lated wave varying as asinusoidal function of the sum of a central angle and a modulation anglerepresenting intelligence, comprising the steps of impressing on thewave a phase shift proportional to a fraction of said modulation angleapproaching unity to produce a treated wave varying as the sinusoidalfunction of the sum of the central angle and a difference anglesubstantially small compared with the modulation angle, deriving fromthe so-treated Wave an electrical potential varying as a sinusoidalfunction of the sum of a given angle andsaid difference angle,impressing on the said potential a local oscillation varying as asinusoidal function of said given angle, deriving from the saidpotential and said local oscillation a resultant electrical potentialsubstantially proportional to the said difierence angle, and convertingsaid resultant electrical potential into an electrical potentialdirectly proportional to the intelligence. 7

2. A method of detecting a frequency modulated Wave varying as asinusoidal function of a central angle and modulation angle representingintelligence, comprising the steps of shrinking the excursion of saidwave by impressing thereon a phase shift proportional to a fraction ofthe modulation angle approaching unity to produce a wave having a shrunkexcursion and varying as a sinusoidal function of the sum of the centralangle and a difference angle having a value substantially smaller than V3. A method of detecting a frequency modu-f lated wave varying as asinusoidal function of the sum of a central angle and a modulation anglerepresenting intelligence, comprising the steps 'of impressing on theincoming wave a phase shift proportional to a fraction of saidmodulation angle approaching unity to produce a treated wave varying asa sinusoidal function of the sum of the central angle and a differenceangle substantially small compared with the modulation angle,heterodyning the so-treated wave with a first local oscillation toobtain an intermediate frequency oscillation varying as the sinusoidalfunction of the sum of an intermediate central angle and said differenceangle, the intermediate central angle being equal to the difierencebetween the central angle of the treated wave and a central anglecorresponding to said local oscillation, impressing on said intermediatefrequency oscillation a second local oscillation varying as thesinusoidal function of saidintermediate central angle, deriving from thesaid intermediate frequency oscillation and the said second localoscillation a resultant electrical potential proportional to saiddifference angle, varying the phase shift impressed on the incoming waveproportional to said resultant electrical potential, and converting saidresultant electrical potential into an electrical potential directlyproportional to the intelligence.

4. In a device for detecting a modulated Wave varying as a sinusoidalfunction of the sum of a central angle and a modulation anglereprerenting intelligence, means for impressing on the incoming wave aphase shift which is a fraction of the modulation angleapproximatingunity to produce a wave varying as a, sinusoidal functionaeoaeeo of the sum of the central angle and a difference angleproportional to and small compared with the modulation angle, means forderiving from the so-treated Wave an electrical potential varying as asinusoidal function of the sum of a given angle and said differenceangle, means for generating a local oscillation varying as a sinusoidalfunction of the said given angle, means for mixing the said potentialwith said local oscillation to produce a resultant electrical potentialsubstantially proportional to the said difference an.- gle, means tocontrol said phase shifting means in accordance With said resultantelectrical potential, and means to convert said electrical potentialinto an electrical potential directly pro portional to the intelligence.

5. In a device for detecting a modulated wave varying as a sinusoidalfunction of the sum of a central angle and a modulation anglerepresenting intelligence, means for impressing on the incoming wave aphase shift which is a fraction of the modulation angle approximatingunity to produce a Wave varying as a sinusoidal function of the sum ofthe central angle and a difference angle proportional to and smallcompared with the modulation angle, means for generating a first localoscillation varying as a sinusoidal function of a heterodyne angledifiering by a predetermined intermediate angle from said central angle,means for mixing said so-treated Wave with said first local oscillationto producean intermediate-frequency wave varying as a sinusoidalfunction of the sum of said intermediate angle and said differenceangle, means for generating a second local oscillation varying as asinusoidal function of said intermediate angle, means for mixing saidintermediate-frequency wave with said second local oscillation toproduce an electrical potential substantially proportional to the saiddifference angle, means to control said phase shifting means inaccordance with said resultant electrical potential, and means toconvert said electrical potential into an electrical potential directlyproportional to the intelligence.

6. A device for detecting a modulated Wave varying as a sinusoidalfunction of the sum of a central angle and a modulation anglerepresenting intelligence, comprising a thermionic tube having anelectron beam generating means, means to vary the intensity of theelectron beam, a collector electrode for the electron beam, and controlmeans to vary the time of transit of the electron beam between aneffective point of origin of the beam and the collector electrodeproportional to and in synchronism with a potential applied thereto,means to apply said modulated wave to said beam intensity varying meansto thereby modulate the intensit of the beam in synchronism with thefrequency of the modulated wave, means to apply to said control means aresultant potential to control the time of transit of the intensitymodulated beam proportional to a fraction of the modulation angleapproaching unity to thereby produce at the collector electrode atreated Wave varying as a sinusoidal function of the sum of the centralangle and a difference angle proportional to and small compared with themodulation angle, means to derive from the treated Wave a potentialvarying as a sinusoidal function of the sum of a given angle and saiddifference angle, a source of a local oscillation varying as asinusoidal function of the said given angle, means to mix the potentialderived from said treated wave and said oscillation and to produce saidresultant potential, and means to derive from said resultant potentialan electrical potential directly proportional to the intelligence.

'7. A receiver for a frequency modulated wave varying as a sinusoidalfunction of the sum of a central angle and a modulating angleproportional to a function of the intelligence to be detected,comprising a thermionic tube having electron beam generating means,means to vary the intensity of the electron beam, a collector electrodefor the electron beam, and control means to vary the time of transit ofthe electron beam between an effective point of origin of the beam andthe collector electrode proportional to and in synchronism with apotential applied thereto, means to apply said modulated wave to saidbeam intensity varying means to thereby modulate the intensity of thebeam in synchronism with the frequency of the modulated wave, means toapply to said control means a resultant potential to control the time oftransit of the intensity modulated beam proportional to a fraction ofthe modulation angle approaching unity to thereby produce at thecollector electrode a treated wave varying as a sinusoidal function ofthe sum of said central angle and a difference angle proportional to andsmall compared to said modulating angle, a narrow band intermediatefrequency amplifier, a frequency changer stage coupled to said collectorelectrode and the intermediate frequenc amplifier to produce in saidamplifier an intermediate frequency wave varying as a sinusoidalfunction of the sum of an intermediate central angle and said difierenceangle, a source of local oscillations varying as a sinusoidal functionof said intermediate central angle, means to combine the output of saidamplifier and the oscillations derived from said source to produce saidresultant potential, and means for deriving from said resultantpotential an electrical potential directly proportional to saidintelligence.

MARC ZIEGLER.

